The present invention relates to digital television (DTV) and, more particularly, to a buffer system for a DTV receiver useful in presenting broadcast data services synchronized to the audio or visual content of a digital television program.
A digital television system includes a transmitter and a receiver which assemble programs comprising audio, video, and ancillary components for coordinated presentation to a user. The transmitter system includes subsystems to receive and compress the digital source data (the elementary or application data streams representing a program's audio, video, and ancillary data components); multiplex the data from the several elementary data streams into a single transport bit stream; and transmit the data to the receiver. At the receiver the transport bit stream is demultiplexed into its constituent elementary data streams. The elementary data streams are decoded and the audio and video data streams are delivered as synchronized program elements to the receiver's presentation subsystem for display as parts of a coordinated television program. The digital television system is described by the ATSC DIGITAL TELEVISION STANDARD, Advanced Television Systems Committee, Doc A/53, 23 Dec. 1997 incorporated by reference herein. The system employs the MPEG-2 systems and video stream syntax of ISO/IEC DIS 13818-1 & 2, INFORMATION TECHNOLOGY—GENERIC CODING OF MOVING PICTURES AND ASSOCIATED AUDIO INFORMATION, International Standards Organization, 1995, 16 Sep. 1995, incorporated by reference herein. Audio coding is in accordance with the DIGITAL AUDIO COMPRESSION STANDARD (AC-3), Advanced Television Systems Committee, Doc A/52, 16 Nov. 1994, 20 Dec. 1995, incorporated by reference herein.
In addition to the basic audio and video content of a televison program, digital television provides the opportunity to augment televison programming with a range of ancillary data services. Ancillary data services described in the ATSC DIGITAL TELEVISION STANDARD include control data; conditional access control data associated with limited access to scrambled signals; and data associated with program audio and video services such as “closed captioning,” emergency broadcasts, and a program guide containing information about the audio, video, and data content of programs being transmitted or to be transmitted in the future.
In addition to these basic ancillary data services, the digital televison standard envisions the broadcast of additional ancillary or private data services. The contents of these services are to be defined in the future by developers of digital television content. These data services can include asynchronous, synchronous, and synchronized streaming and non-streaming data. Synchronized streaming and non-streaming data afford the opportunity to provide data services that are associated with a particular audio or visual instant of a television program. The suite of ATSC digital television standards will include a specification for data broadcasting drafted by the T3S13 working group of the Advanced Television Systems Committee. This ATSC DATA BROADCAST SPECIFICATION, currently in draft form, is expected to include data and system specifications to facilitate data transmission in a manner compatible with the digital multiplex bit streams of ISO/IEC 13818-1 (MPEG-2 Systems) used in digital television video broadcasting. The specification is also expected to describe mechanisms necessary to permit ancillary data to be associated with particular audio or visual content in a television broadcast.
The ATSC DIGITAL TELEVISION STANDARD describes a model for a hypothetical ancillary services target system decoder for a DTV receiver. This idealized decoder is modeled after a conceptual model for a transport stream system target decoder (T-STD) described in the ISO/IEC 13818-1:1996 (MPEG-2) standard. The MPEG-2 conceptual T-STD model comprises an individual system target decoder for each of the elementary data streams representing the video, audio, and system information components of a television program. However, the MPEG-2 system specification does not define a complete system target decoder for ancillary, synchronized data services.
The ATSC model ancillary services target system decoder includes a buffer system linking the receiver's transport stream demultiplexer with the application decoder for the specific type of elementary data stream being decoded. The transport stream demultiplexer separates and reconstructs the several elementary streams from the single multiplexed MPEG-2 transport bit stream. The application decoders decode the data elements or access units making up the data elementary streams in preparation for presentation to the user. The ATSC model ancillary services target decoder, as well as the decoder described in the European Digital Video Broadcasting Consortium (DVB) specification, are two buffer systems with a 512-byte transport buffer followed by a smoothing buffer.
For synchronized presentation of streaming data the DTV receiver must be able to collect the packets of data, reassemble them into data access units (an elemental unit of data to be decoded and presented in association with an instant in a video or audio bit stream), and send them to the application decoder that is processing the data and converting the data to a form suitable for presentation at the desired time instant. If the receiver receives the data earlier or at a faster rate than required, the excess data must be stored in a buffer. On the other hand, sufficient data must be readily available in the receiver or the presentation will not be smooth and properly synchronized to the video or audio content.
The ATSC model for an ancillary service target decoder consists of a transport demultiplexing buffer and a smoothing buffer. However, there is no buffer for reconstructing data access units to be decoded and then presented in synchronization with the audio or visual elements of a program. Likewise, the size of a data access unit, the frequency of transmission of data access units, and the method of synchronization is not established by the ATSC DIGITAL TELEVISION STANDARD and not contemplated by the MPEG-2 specification. Derivation of the general T-STD model for all kinds of ancillary data is not straightforward because the decoding model for ancillary data can be anything from simple to complex. As a result, the quantity of data the receiver may be required to cache might exceed its capacity (a condition of memory overflow) or throughput capabilities. Alternatively, the receiver may fail to receive all of the data in a data access unit in time for decoding and synchronized presentation with a specified instant in the audio or video data streams resulting in a loss of data and inconsistent performance (a condition of memory underflow).
What is desired, therefore, is a buffer system for an ancillary service target decoder for synchronized data services that establishes the size limits of a data access unit for those services and enforces a transmission schedule on the MPEG-2 multiplexer of the transmitter or emission station such that the data arrives in time for proper synchronized presentation but in a quantity and at a rate which will not overwhelm the receiver so no data is lost.